The 5-HT4 receptor is widely distributed in the body, in the periphery as well as in the central nervous system. In the periphery it is found in the gastrointestinal tract, for example in the esophagus (Moummi et al., 1992), the ileum (Buchheit and Buhl, 1991) and colon (Elswood et al., 1991). It is also present in the atrium (Kaumann. et al., 1990), the bladder (Candura et al., 1996) and the adrenal glands. In the rat brain, 5-HT4 mRNA has been discovered by in situ hybridization in the olfactory tubercle, the striatum and the hippocampus (Vilaro et al., 1996). The wide distribution in different tissues of the 5-HT4 receptor is parallelled by a wide variety of 5-HT4 variants caused by alternative splicing of exons. The splice variants described so far (Gerald et al., 1995; Claeysen et al., 1996; Van den Wyngaert et al., 1997; Claeysen et al., 1997; Blondel et al., 1997; Blondel et al., 1998) are all variations of the cytoplasmic C-terminus.
The predicted protein structures encoded by cDNA sequences already known reveal seven transmembrane domains for the complete open reading frames. In addition to their structure and 5-HT4 receptor coupled signal transduction events (increase in cAMP formation, opening of K+ channels), 5-HT4 receptors have also been classified as G-protein coupled receptors (GPCRs).
The present inventors have identified a novel human 5-HT4 splice variant (h), which leads to the insertion of 14 amino acids into the second extracellular loop of the receptor protein. They expressed the isolated full length cDNA transiently in mammalian cells in order to compare its pharmacology with already known 5-HT4 splice variants and its tissue distribution is analyzed by RT-PCR.
Therefore, according to a first aspect of the present invention, there is provided, an isolated substantially pure form of nucleic acid molecule encoding a human 5-HT4(h) receptor. Preferably the 5-HT4(h) receptor encoded by said nucleic acid molecule comprises the amino acid sequence illustrated in FIG. 1B or a functional equivalent, derivative or bioprecursor of said receptor.
Thus, the present invention comprises a nucleic acid molecule encoding a human 5-HT4(h) receptor or an immunologically and/or biologically active fragment thereof, which comprises a nucleotide sequence selected from the group consisting of:                (a) nucleotide sequences encoding the amino acid sequence depicted in FIG. 1B;        (b) nucleotide sequences comprising the coding sequence as depicted in FIG. 1A;        (c) nucleotide sequences encoding a polypeptide derived from the polypeptide encoded by a nucleotide sequence of (a) or (b) by way of substitution, deletion and/or addition of one or several amino acids of the amino acid sequence encoded by the nucleotide sequence of (a) or (b);        (d) nucleotide sequences the complementary strand of which hybridises with a nucleotide sequence of any one of (a) to (c);        (e) nucleotide sequences encoding a polypeptide the amino acid sequence of which has an identity of 70% or more to the amino acid sequence of the polypeptide encoded by a nucleotide sequence of any one of (a) to (d);        (f) nucleotide sequences encoding a polypeptide capable of binding a ligand of 5-HT4(h) comprising a fragment or an epitope-bearing portion of a polypeptide encoded by a nucleotide sequence of any one of (a) to (e);        (g) nucleotide sequences comprising at least 15 consecutive nucleotides of a nucleotide sequence of any one of (a) to (f);        (h) nucleotide sequences comprising a nucleotide sequence which is degenerated as a result of the genetic code to a nucleotide sequence of any of (a) to (g).        
Advantageously, the isolated nucleic acid according to the invention may be used for expression in, for example, a host cell or the like using a suitable expression vector. Preferably, the nucleic acid may be a DNA molecule or a cDNA molecule. Preferably, the DNA molecule has the nucleic acid sequence as illustrated in FIG. 1A.
The nucleic acid molecule is preferably capable of hybridising to the sequences of the invention under conditions of high stringency or to the complement thereof.